I have a really odd problem here - ASP.NET 3.5 Webforms app on IIS 6.

The effect is that the user connects to our site, and gets an ASP.NET session, enters some data and suddenly, all his data entered (and stored in the session) is gone.

Error logs shows us that for some odd reason, he's just getting a new session in the middle of working in our app.

From the IIS logs, we see that within a single ASP.NET request, the user agent reported from the user's browser switches - from MSIE+7.0 to MSIE+8.0.... how can that be?

Excerpt from log:

07:06:38   GET /SomePage.aspx  80 - x.x.x.139   Mozilla/4.0+ (compatible;+MSIE+7.0;+Windows+NT+5.1)   401 
07:06:38   GET /SomePage.aspx  80 DOMAIN\USERNAME  x.x.x.139  Mozilla/4.0+ (compatible;+MSIE+7.0;+Windows+NT+5.1)   200

07:06:39   GET /javascript/somefile.js 80   DOMAIN\USERNAME x.x.x.139   Mozilla/4.0+ (compatible;+MSIE+8.0;+Windows+NT+5.1)  200 
(lots more requests for .css, .js, .gif, .jpg - all with MSIE+8.0 ....)

It seems the two requests to the .aspx page are done in MSIE+7.0 mode, while any subsequent requests for CSS and JS files as well as GIF und JPG graphics report back MSIE+8.0 ...... WTF?!?!?

Not sure if that really is the root cause of the sudden loss of ASP.NET session - but that user agent switching in itself leaves us scratching our heads.... any ideas?

If this behavior is not the root cause of those "lost sessions" - any ideas / leads as to what could be the cause there? I haven't been able to dig up anything overly useful so far from here, Bing, Google or any other source....

Update: I read in this forum thread that the fact the user agent is different between the first GET (which fetches the .aspx page) and the subseq

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